Any fabric-type rooftop cargo bag, including the Rightline Sport 3 XL Waterproof Carrier # RL100S30 that you referenced, will use integrated straps to secure to your vehicle's roof rack or side rails. While it is possible to add a cable-style lock such as # e98891, this would only slow down any would-be thief. Since the bag's straps can be cut the lock would only provide for a modest complication for a person intent on stealing your bag and property.
We do offer lockable cargo carriers... view full answer...